Fix localization of polls

[lukasrad02]

The polls feature needs better integration into page localizations:

• Allow localizing options and option descriptions
• Don't treat translated polls as separate polls
  • A user who voted on a German poll should not be able to vote on the corresponding English poll and vice versa
  • Results for a poll should take votes from the German and English variant into account
• Synchronize properties like the eligible groups (see Keep page fileds in sync with translated pages #373 and the fix in Make all ParentalManyToManyFields synchronized #491)